Document
9 - by 15 - Foot Low Speed Wind Tunnel
Acoustic Improvements
Expanded Overview
Acoustics Branch Point - of - Contact: David Stephens
April 2016
Executive Summary
The 9 - by 15 - Foot Low Speed Wind Tunnel (9x15 LSWT) at NASA Glenn
Research Center was built in 1969 in the return leg of the 8 - by 6 - Foot
Supersonic Wind Tunnel (8x6 SWT). The 8x6 SWT was completed in
1949 and acoustically treated to mitigate community noise issues in
1950. This treatment included the addition of a large muffler
downstream of the 8x6 SWT test section and diffuser.
The 9x15 LSWT was designed for performance testing of V/STOL
aircraft models, but with the addition of the current acoustic
treatment in 1986 the tunnel been used principally for acoustic and
performance testing of aircraft propulsion systems. The present
document describes an anticipated acoustic upgrade to be completed
in 2017 .
8x6/9x15 Facility Manager : David Stark
Unique facility for testing propulsors
9x15 LSWT 8x6 SWT Mach 0 to 0.23 0 to 0.1 Number 0.25 to 2.0 Simulated Sea Level 1000 to 35,000 ft Altitude
Fan/Propulsor Testing in 9x15 Tunnel
• The GRC 9x15 Low Speed Wind Tunnel has
Honeywell Turbofan with Acoustic Liners
been extensively used to study and acoustically
characterize nearly all of the NASA/Industry
propulsor concepts over the past 20 years.
• Except for maintenance, the acoustic
treatment has remained essentially unchanged
in 20+ years.
NASA/GE Open Rotor P&W Geared Turbofan P&W Advanced Ducted Propulsor
Testing quiet fans requires
a quiet wind tunnel
• Historically this has been done by testing
a Mach 0.1, which is below true take - off
and landing speeds.
Empty 9x15 vs Low Power ADP with Liners Empty 9x15 vs Low Power ADP with Liners Empty 9x15 vs Low Power ADP with Liners
• Future fans may be even
90 90
quieter
80 80 80
• Low tip speed
• Low pressure ratio
70 70 70
• Acoustic liners
• Open rotors and other
60 60 60 ADP Aft (126º emitted)
Third Octave SPL, dB concept fans require
Third Octave SPL, dB Third Octave SPL, dB ADP Aft (126º emitted) - - ADP Aft (126º emitted) - ADP Broadside (89º emitted)
testing at higher tunnel
ADP Broadside (89º emitted) ADP Broadside (89º emitted) One 50 One 50 One 50 ADP Forward (23º emitted)
speeds than Mach 0.1
ADP Forward (23º emitted) Current Empty 9x15, Mach 0.10 ADP Forward (23º emitted) Current Empty 9x15, Mach 0.10 40 40 Current Empty 9x15, Mach 0.20 100 1000 10000 100000 100 1000 10000 100000 100 1000 10000 100000 Frequency, Hz Frequency, Hz Frequency, Hz
Support from NASA Agency and Center Levels
• Preliminary assessment by Jacobs Technology, Inc performed in 2012
• Funded by Environmentally Responsible Aviation
• Initial FY15 NASA funding resulted in initiation of 9x15 Design
contract that resulted in completion of a 30% design review.
• Additional funding in FY15 from NASA GRC, NASA AETC project and
NASA Augmentation funded a 9x15 Design/Build contract which
began in September 2015 from the 30% design point. The FY15
funding provided 60% of the total project Design/Build cost.
• FY16 funding from NASA GRC and NASA AETC is currently being
approved by Congress through the FY16 NASA Operating Plan and
this funding will provide the remaining 9x15 Design/Build funding .
Anticipated Schedule
• December 2015 – Final pre - construction aerodynamic calibration of
8x6 and 9x15 test sections
• January 2016 – Final pre - construction background noise
measurement of 9x15 test section
• September 30, 2016 – Boundary layer ingestion test in 8x6 tunnel
ends
• October 31, 2016 – Wind Tunnel Shutdown for Start of Site Work
• Both 8x6 and 9x15 shut down
• September 4, 2017 – Acceptance Testing Begins
• Acceptance testing for 8x6 and 9x15 test sections
• September 29, 2017 – Tunnel Ready for Testing
Planned Wind Tunnel Renovation
Complementary but discrete improvements
1. Add fairings and turning vanes to turn 2
2. Add acoustic baffles downstream of doors 1 & 2
3. Replace test section flow surfaces, remove slots
4. Reshape diffuser and add acoustic treatment
5. Add turning vanes to turn 3
1 2 5
Flow
9x15 Tunnel, Top View
Turn 2 Turning Vanes
Cooler
Muffler
Exit
Improvements to Turn 2
Velocity Field in Turn 2
Heat Muffler Exchanger Exit
Case 1: Existing design
Case 2 : Proposed C - 2 design
Proposed designs provides much
more uniform airflow into cooler.
This should result in more uniform
temperature in the test section.
Downstream of Cooler
Looking upstream at the Cooler
Test Section
Looking Downstream from Inlet
Current 9x15
Test Section
• The test section surface is
perforated steel over a bulk
Kevlar absorber
• Facing plate is 16 ga , 1/8”
holes, 40 percent open
• The acoustic treatment is
built as dozens of individual
boxes
• There are slots in both walls,
and many seams throughout
the tunnel
• The 2012 study by Jacobs
concluded that the majority
of the noise in the 9x15 test
section above 2 kHz is due to
boundary layer flow over
perforated steel surface
Test Section Perforated Metal (Current)
Jacobs concluded that the noise in
From Summer
the 9x15 test section above 2 kHz is
due to airflow over the perforated
2012 Study
steel tunnel walls
90 90 Prediction:
~ 7 dB reduction
80 80 70 70 60 60 50 50 Measured In-Flow SPL Measured In-Flow SPL NASA 9x15 SPL Goals in Test Section NASA 9x15 SPL Goals in Test Section 40 40 BLNoise Estimate (roughness = 1.5mm) BLNoise Estimate (roughness = 1.5mm) BLNoise Estimate (roughness = 0.1mm) 1/3rd Octave Band Sound Pressure Levels (dB) 1/3rd Octave Band Sound Pressure Levels (dB) 30 30 100 1000 10000 100000 100 1000 10000 100000 Frequency (Hz) Frequency (Hz) Front
Proposal to improve
the 9x15
• Jacobs estimated a 7 dB
reduction in roughness noise
is possible by replacing the
current wind tunnel surface
with a woven glass fiber
cloth, lowering roughness
from 1.5mm to 0.1mm (not
including seams)
• It is a synthetic fabric made of
flat weave bonded to a
coated perforated metal
sheet
• Sample shown
Back
Roughness Noise Facility
at Virginia Tech
Inside Virginia Tech Facility 2014
Test section modification
successful, allowing
flush mounted samples
with deep acoustic
treatment below.
Extensive roughness noise testing at Virginia
Tech to find quiet surface
• More than 30 samples tested
Substantial noise benefit possible by
• Combinations of
replacing current test section flow surface • Perforate (hole size, % open, thickness) • Covering (glass cloth, wire mesh)
10 dB
PSD #6 PSD,
-10 -5
9x15 Baseline Mic -10 Solid Plate
-20 -15
5/32 63% 200x600 -20 0 0.5 1 1.5 2 2.5 Frequency, Hz x 10
-30
0 5000 10000 15000 20000
• Very wide range of results
Frequency, Hz
• For more information, see AIAA - 2015 - 3261 21
Diffusion Bonded Plates
Diffusion bonded plates (DBP) are produced by combining perforated sheet metal with fine
wire cloth. These materials are joined in a vacuum furnace under heat and mechanical
load in a process called diffusion bonding.
The function of the DBP is to allow passage of sound waves into bulk absorber material, while producing minimal self - noise under grazing flow.
DBP Details:
• 5/32” holes on 3/16” centers
(63% open area) 16 - gauge
perforated sheet metal
• 200x600 Twilled Dutch Weave
wire cloth
• 304 stainless steel cloth and
plate
• Wire cloth “dimples”
(depressions over perforations)
approx. 0.005” deep
• Flow resistivity ~12 CGS Rayls
Goal: Minimal impact to anechoic quality
• Lower self noise AND maintain anechoic quality
• Impact of wire cloth over perforate has been investigated,
examples given in next four slides
• LaRC Normal Incidence Tube
• Riverbank Acoustical Laboratory
• Glenn Acoustical Testing Laboratory
• LaRC Curved Duct Test Rig
• Testing of reinforced panels in GRC Acoustics Lab is pending
• Anechoic quality of test section to be assessed by external
contractor according to ISO 26101 during July 2016
NASA Langley Normal
Incidence Tube
• Established facility
• Built for liner testing
• Limited to 3000 Hz, plane waves
• Ripples due to depth of bulk absorber
1.00 0.95 0.90 0.85 Absorption Coefficient Current 9x15 Perforate Proposed DBP
Langley Normal
0.80
Incidence Tube
0 500 1000 1500 2000 2500 3000 3500 Frequency, Hz
Reverberation room test at Riverbank
Acoustical Laboratory
Six configurations were tested, with variation in bulk absorber density, with and without
perforated panel covering and with different panel rib arrangements.
Reverberation Room Test Results 1.10 1.00 0.90 0.80 Normalized Acoustic Absorptivity higher density composite higher density composite + DBP higher density composite + DBP + solid ribs higher density composite + DBP + perf ribs 0.70 10 100 1000 10000 1/3 OB Frequency, Hz The DBP causes < 3% absorption reduction above 400 Hz.
Reflection Test at NASA Glenn
Acoustical Testing Laboratory
Pressure, Pa -5
• Not limited in frequency range
-10 0 0.005 0.01 0.015 0.02
• Accurate positioning a challenge
Time, Seconds ATL Reflection Test Results 0.98 0.96 0.94 0.92 0.9 0.88 Current 9x15 0.86 Proposed Panel Absorption Coefficient 0.84 10 per. Mov. Avg. (Current 9x15) 0.82 10 per. Mov. Avg. (Proposed Panel) 0.8 0 1000 2000 3000 4000 5000 6000 7000 8000 Frequency, Hz Attenuation of 9x15 Samples Measured at CDTR, Mach 0.20 BaselineM02D00 BaselineM02D01
NASA Langley
BaselineM02D30 WireClothM02D00 WireClothM02D01
Curved Duct Test Rig
WireClothM02D30
• Engine liner test with flow
Insertion Loss, dB
• Rectangular duct modes
• Limited to 3000 Hz
0 500 1000 1500 2000 2500 3000 Frequency, Hz
Diffuser
Looking upstream into the diffuser
Recirculation
Existing
Attached Flow
Modified
Test Section to be lengthened 5 - feet into diffuser
Red Lines denote limits of measurements in upstream and downstream directions. First or last measurement repeated as needed to complete analysis.
• Current test section
length restricts aft
measurement angles
• Floor mounted
microphones are a
marginal solution
• This can have a
significant impact on
EPNL calculations
• The addition of a 5 - foot
straight extension into
the current diffuser will
enable measurement
to 150º geometric from
upstream
Anticipated Improvement to 9x15 noise levels
after all 5 upgrades
90.0 Empty Tunnel, Jacobs 80.0 Frequency (2012) Prediction 630 81.4 65.6 794 69.3 80.8 1000 71.9 80.6 70.0 1260 82.5 73.3 1587 80.8 73.7 2000 78.7 73.9 60.0 2520 79.7 73.1 Third Octave SPL, dB - 3175 81.5 73.5 4000 83.5 73.3 One 5040 85.1 72.7 Current Empty 9x15, Mach 0.20 50.0 6350 85.3 72.4 8000 70.5 84.5 Predicted Empty 9x15, Mach 0.20 10079 68.3 83.1 12699 81.7 64.7 40.0 16000 80.6 63.0 100 1000 10000 100000 20159 79.1 61.3 Frequency, Hz 25398 77.9 58.5 32000 75.1 55.7 Acceptance criteria: No less 40317 71.7 52.6 50797 71.3 49.9 than 3 dB from prediction
Relative to P&W Advanced Ducted Propulsor (ADP)
Model Fan Measurements
Empty 9x15 vs Low Power ADP with Liners Third Octave SPL, dB - ADP Aft (126º emitted) One ADP Broadside (89º emitted) ADP Forward (23º emitted) Current Empty 9x15, Mach 0.20 Predicted Empty 9x15, Mach 0.20 100 1000 10000 100000 Frequency, Hz
Summary
• GRC responding to industry feedback on 9x15 background
noise level requirements
• Design and build contract in place to implement changes to
9x15 for improvements and funding identified and going
through approvals
• The work to date by suggests substantial reductions in
background that will improve signal - to - noise required in
future systems
• Additional work on measurement and signal processing are
expected to create additional signal - to - noise headroom
Additional Materials
Noise Predictions for Other Mach Numbers
Current Predicted 90.0 Mach Mach Mach Mach Mach Mach Frequency 0.10 0.15 0.20 0.10* 0.15* 0.20 80.0 630 62.4 73.1 81.4 53.3 62.1 65.6 5 Pa - 794 62.4 73.1 80.8 55.2 64.4 69.3 1000 62.5 73.6 80.6 57.6 65.2 71.9 70.0 1260 64.3 74.1 82.5 57.7 66.3 73.3 1587 65.7 73.1 80.8 57.8 67.1 73.7 2000 67.1 73.6 78.7 57.0 66.7 73.9 60.0 2520 67.9 75.3 79.7 55.6 67.0 73.1 3175 67.9 76.8 81.5 54.9 66.3 73.5 4000 67.3 77.9 83.5 53.3 65.5 73.3 50.0 Current, Mach 0.20 5040 66.1 78.3 85.1 51.3 64.9 72.7 Third Octave Band SPL, dB ref 2e Current, Mach 0.15 - 6350 63.8 77.3 85.3 46.8 62.9 72.4 Current, Mach 0.10 40.0 One 8000 62.5 75.6 84.5 44.8 60.4 70.5 Predicted, Mach 0.20 10079 60.8 74.4 83.1 42.9 56.5 68.3 *Predicted, Mach 0.15 *Predicted, Mach 0.10 12699 58.2 73.4 81.7 38.8 55.1 64.7 30.0 16000 55.8 71.7 80.6 36.3 53.0 63.0 500 5000 50000 20159 54.1 69.5 79.1 34.9 49.5 61.3 Frequency, Hz 25398 51.7 67.7 77.9 47.1 58.5 32000 48.1 65.0 75.1 44.5 55.7 40317 44.0 61.3 71.7 52.6 50797 42.1 57.3 71.3 49.9 * Provided prediction is for Mach 0.20, scaled to other Mach numbers by NASA. The prediction was scaled in frequency by the Mach number ratio and in amplitude to mimic the current noise.